This invention relates to transfer cases for four wheel drive systems, and more particularly to a full time four wheel drive transfer case.
Four wheel drive systems have been in existence for many decades. The output of the engine was split between the front wheels and the rear wheels by a transfer case. One type of system used for many years provided undifferentiated power to all wheels of the vehicle. This gave the vehicle good mobility under adverse surface conditions. However, this type of drive was not particularly useful for normal highway driving. Since all four wheels operated at the same speed the vehicle could not be turned without sliding or scuffing one or more tires. This drawback resulted in the development of systems which included a conventional two wheel drive mechanism. For highway use the two wheel drive was utilized and this caused the vehicle to assume the characteristics of most other two wheel vehicles.
It is desirable in a four wheel drive vehicle to obtain the benefits of differentiation provided in conventional two wheel drive vehicles while also obtaining the benefits of a conventional four wheel drive, when desired. Commonly assigned copending patent application Ser. No. 409,342, now U.S. Pat. No. 3,848,691 describes a transfer case adapted to provide such characteristics. The transfer case shown in the aforenoted patent was actuated manually to lock out a differential unit when the differentiation was not desired.
It has been proposed that the manual operation be replaced by automatic locking apparatus. One type of automatically operated apparatus developed utilizes electronic sensing apparatus which senses a difference in rotation speeds of two rotatable speeds of two rotatable parts of the differential. When the difference in speeds exceeds a predetermined value, a solenoid is actuated to operate a mechanical clutching mechanism which couples the two rotatable parts together and renders the differential ineffective. Co-pending commonly assigned U.S. application Ser. No. 331,456, filed Feb. 12, 1973, now U.S. Pat. No. 3,845,671 shows such apparatus.
Another type of automatic apparatus includes a viscous coupling having a plurality of plates connected to one rotatable member and interleaved with a plurality of plates connected to another rotatable member. Viscous fluid substantially fills the housing containing the interleaved plates. As the speed difference between two rotating parts increases, the viscous fluid is sheared by the interleaved elements, which results in a tendency to interlock the rotatable parts.
The present invention relates to apparatus which includes both mechanical and viscous coupling features.